Prepare 4.2.1 release

Author: HexDecimal

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
+## [4.2.1] - 2023-07-28
 ### Fixed
 - Unpickled worlds had reversed relations from what were saved.